VideoLAN's VLC vide player for Windows 8 has been released
at the Windows Store, although still in a beta form.
VLC is a very popular open source video player but VLC for Windows 8 Beta offers even more, as it is a complete multimedia app that lets you manage and play your video files and your music files. You get dedicated "panels" in the app to see all the videos and songs on your device and supports all the same file types as the desktop version of VLC for Windows - including Ogg, FLAC and MKV.
If you can bare with a few bugs and kinks that still have to be worked, just give it a try.